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Calamiteitenroutes and U-routes
Calamiteitenroutes in Belgium and U-routes (Uitwijkroutes) in the Netherlands are permanent routes that are used to reroute traffic in case of a closure of the main road.
The road type should be at least Minor Highway.
The Belgium Calamiteitenroutes are marked with a single character, like F.
More information on Calamiteitenroutes see the Belgium site wegenenverkeer.be.
The Dutch routes are marked with an "U" plus number, like U27.
More information on U-routes can be found in the Dutch Wegenwiki.